In a typical radio system, information is modulated onto a radio carrier by a transmitter. This signal then travels via an unknown and changing environment to the receiver. The ability to estimate the characteristics of this propagation environment and to mitigate the impact on the received signal is often key to the performance of a receiver.
FIG. 1 depicts various processing stages that form part of such an approach. It should be noted that the blocks shown in FIG. 1 represent processing operations performed on a received signal but do not necessarily correspond directly to physical units that may appear within a practical implementation of a receiver. The first stage 101 corresponds to the radio frequency processing. During the radio frequency processing, the received signal is down-converted to base-band using a mixer 103. The reference frequency used to drive the mixer is generated by an oscillator 104. Following this carrier down-conversion, the signal is low-pass filtered 102 and then passed to the mixed-signal processing stage 108. The mixed signal processing includes Analogue-to-Digital Conversion (ADC) 105, sampling 106 and low pass filtering 107. The resulting signal, which is now digital, is supplied to the digital signal processing stage 111 where it is processed such that the transmitted information can be recovered. The received signal is first processed by the channel estimation unit 109 where an estimate of the Channel Impulse Response (CIR) is generated. This estimated CIR is processed in combination with the received signal by the demodulation unit 110 such that the sequence of transmitted bits can be recovered.
In the downlink of cellular communication systems, a pilot signal is usually transmitted in combination with the information bearing signals such that the receiver can estimate the propagation channel. For Wideband Code-Division Multiple Access (W-CDMA) schemes, this pilot signal is typically code-multiplexed with the transmitted signal. For example, in the 3GPP standard, the Common Pilot Channel (CPICH) is a sequence of known bits which are modulated, spread and added to the downlink signal (3GPP TS 25.211; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD)). At the receiver, it is possible to generate an estimate of the CIR by correlating the received signal with the known CPICH pilot sequence.
The accuracy of the channel estimation process is crucial in determining the quality of the demodulation process. For W-CDMA systems, it is typical to use a Rake architecture at the receiver (CDMA—Principles of Spread Spectrum Communication, Andrew J. Viterbi, Addison-Wesley Wireless Communications Series). In the Rake receiver, the weights associated with the different fingers correspond to the estimated CIR taps at the finger delay locations. The noise affecting these finger weights increases the likelihood of errors in the demodulation process. More recently, new receiver architectures have been introduced where the demodulation accuracy is improved at the expense of the implementation complexity. The Linear Minimum Mean Square Error (LMMSE) equaliser is an example of such an architecture (Chip-Level Channel Equalization in WCDMA Downlink, K. Hooli, M. Juntti, M. J. Heikkila, P. Komulainen, M. Latva-aho, J. Lilleberg, EURASIP Journal on Applied Signal Processing, August 2002). The LMMSE equaliser improves the performance of the demodulation unit by mitigating the distortions introduced by the propagation channel. The LMMSE equaliser can be implemented using a pre-filter Rake architecture (Equalization in WCDMA terminals, Kari Hooli, PhD thesis, 2003) where the conventional Rake receiver is preceded by a linear filter which aims to remove the Inter-Symbol Interference (ISI) introduced by the channel. In the pre-filter Rake receiver, the channel estimates are used both to set the weights of the Rake receiver as well as to derive the coefficients of the linear pre-filter.
For the pre-filter Rake to operate correctly, it is important that the estimated channel impulse response is wide enough to cover all the paths with significant power. This calls for a large window of channel estimation. However using a large window has three drawbacks. First, the wider the estimation window, the larger the number of computations required to generate the channel estimated values. The second problem is that a larger channel delay spread requires more taps in the pre-filter for accurate equalisation. Finally, the number of computations required to calculate the pre-filter configuration typically increases with the square of the number of channel taps. Hence, it is important to keep the length of the channel to be equalised as small as possible while still being able to react quickly to paths appearing at different locations.